I'm not well - versed in the 'slow frontier light novel' so I can't give a definite main theme. But thinking about it, 'frontier' can symbolize a place of new opportunities and threats. It could be about the characters' struggle to adapt to this new environment. The 'slow' part might suggest that the story takes its time to show how the characters grow and change in response to the frontier. It might also touch on themes like perseverance in the face of difficulties in this new frontier, and how they slowly but surely make a mark in this uncharted territory.
Since I'm not sure which 'slow frontier light novel' you specifically refer to, it's hard to say. But generally, 'frontier' in a title might suggest themes related to exploration, new beginnings, or the challenges of living in a less - developed or uncharted area. 'Slow' could imply a more relaxed pace of the story, perhaps focusing on character development over rapid plot progression.